The general idea of how and why the Reinforced Cement Concrete (RCC) elements are placed in structure will help consumers understand the design of their structures.

The damage caused by the Gorkha Earthquake in 2015 has made the people of Nepal realize the importance of strong buildings. People are now taking help from experienced architects and engineers from the initial phase of the construction all the way to its completion.  But it is also our job to understand the basics of what the engineers and architects are doing to make our buildings safe and strong.

When it comes to a building’s rigidity, the structural elements play the key role.  Reinforced Cement Concrete (RCC) structures are mostly used in today’s construction in Kathmandu.  There are many types of structures, such as: load bearing, where the main load is taken by the brick wall; timber structures, where pillars, beams and floorings are made of woods; steel structures, where heavy sections of steel are used as the major structural elements, and bamboo structures.  In Kathmandu, RCC buildings are more common new buildings than any other types of buildings.

Any building comprises of foundation, columns, beams, and slabs as major structural elements.  These elements help transfer the live and the dead loads to the soil beneath.  The live loads are the occupants of the building, and the dead loads are the things that we put on the building such as furniture, walls and partitions, floorings, decorations etc.  These loads initially taken by the slab get transferred to the beams, then to the columns, and then unto the foundation.  Hence as the columns are the major structural elements to transfer loads, they are comparatively larger in size than the beams.  If any building comprises of beams larger than the columns then it is not designed properly.

Let us have a look at the major topics of RCC structures which will help any layman understand the basic design made by structural engineers.

Stirrups: Stirrups are the most basic and the most important members. These are binder rods made by the 7-8 mm dia bars.  They protect bars from bulging out during flexure also they help to carry out vertical shear force and resist diagonal crack.  The stirrups are spaced at equal intervals of 4 inches in foundation, and in the area of column lapping. In other areas the stirrups are placed according to the structure design.   The stirrups are also called the “rings”.  Double stirrups or the double ring design is compulsory in columns whereas single stirrups are placed in beams.  The end of the ties of stirrup should be bent at 135 degrees which prevents opening of loops and consequently buckling.

Foundations: The main core strength and load of the building will be taken by its foundation, so it’s necessary to design a good foundation.  It is better if the owner asks the structure engineer to design the foundation of required storey + 1 i.e. if you are building a 2.5 storey building then the structure engineer should design the foundation for 3.5 storey.  There are different kinds of foundation like isolated, combined, raft, strap footing, pile, combined etc.  For a residence, raft and piles are in general needed but isolated, combined and strap footing combinations are more widely used.

Isolated footings: It is also known as Haati paila (elephant foot) are the ones with a square base of generally 5 feet by 5 feet which tapers to the height of a minimum of 1’6 inches, thus giving the form of a half pyramid, and the pillar is at the center of the square.  The pillars of the foundation have a long wide foot which is placed on a net of bars, and the half pyramid form is created by casting with an accurate ratio of cement, sand and concrete.  The pillar rods are left alone and cast later.

Combined footing: It is a combination of two foundations, with one double net at the bottom and the other at the top of the foundation.  These are usually placed when two isolated footings are near.  First the net is placed on the p.c.c, then the column skeleton is placed, and then the top net, which is cast at the height of a minimum of 1’6 inches.  The size of the combined footings is rectangular.

Strap footing: A footing which consists of beams at the bottom most part which are connected to other footings is known as strap footing. They are usually connected to the eccentric footing or other footings for rigidity. As the beams are connected to the other footings, they all functions as one.

Eccentric footing: These are the isolated footing with only three sides and can be seen on the property line of the land,. This type of footing should be connected with other footings.

Tie beam: After the foundation has been cast the lower tie beams are tied just at the top of the foundation. These beams connect all the foundation underneath the soil and are usually of 9 inch by 9 inch (can be greater according to the design) size.  After these and the pillars are cast and the dwarf walls are erected on these tie beams above the desired ground level, yet another tie beam, which is known as the upper tie beam or the plinth level beam is cast. This level is called the plinth level.

Columns: Columns are the vertical structures and are also known as pillars. They transfer the loads from the beam to the foundation; and they are erected from the foundation to the height of the building. A column consists of vertical bars which is tied with stirrups and later concreted. A single bar cannot cover the whole height of the building hence we have to join and overlap the bars floor wise. The overlapping of the bars should be in the mid-half of the column, and the overlapping should be done at H/6 i.e. overlapping should not be less than 1/6 of the total height.  The spacing of the stirrups should be close in these regions, and in the beam and column joints.  The Nepal building Code has stipulated that the column size needs to be minimum of 1 foot by 1 foot.

The diameter of bars used in columns is designed by the structural engineers and it varies from 16 mm dia to 20 mm dia for residential buildings. The stirrups of 8 mm dia are usually used in columns.

Beams: These are the horizontal structural members which transfer the load on slabs to the columns, and horizontal bars are placed according to the design and are held together by the stirrups. The stirrups should be placed closely near the beam column joint and according to the design in the rest of the area. There are generally main bars that go throughout the beam, and cutout bars called top and bottom bars. The lapping in the beam should be avoided but if needed it should be near the column joints. The top end of the bar should be inserted on the column beneath and at the same area the bottom end of the bar should be faced upwards for the upcoming column, which will help lock the entire set of beams and columns.

Slab: The slabs are the floors of the building, and usually have a thickness of 5 inches and above. There are various types of slab such as corrugated, ribbed, waffle, one way slab, two way slabs etc. In Kathmandu, the use of two way slabs is more common in residence where the main reinforcements are provided in both the direction, The top bars of slabs are placed according to design. Generally 10 mm dia and 8 mm dia bars are used for slabs.

This general information on the structural element of RCC buildings should be followed in order to construct a safe building, often due to lack of knowledge, contractors tend to carry out their work according to their own convenience which sometimes can be dangerous in the long run. The consumer should also ask site supervising engineers or architects to explain to the contractors before they start their work, in order to avoid redoing of the work if there are any mistakes.